In a laser printer including a rotary developing unit, a plurality of developing cartridges storing different color toners are set in the rotary developing unit. At the printing time, while the rotary developing unit is rotated, the developing cartridge storing the necessary color toner is brought adjacent to a photoconductor drum, and the toner is supported on a latent image formed on the photoconductor drum and is transferred from the photoconductor drum via a transfer belt to a sheet for fixation.
Since the toner stored in the developing cartridge is consumed gradually, a negative pressure may be developed in the developing cartridge because of a decrease in the volume occupied by the toner. In contrast, it is also possible that the pressure in the sealed developing cartridge may rise because of the effect of heat from a heating element positioned in the proximity of the developing cartridge. If the difference between the pressure in the developing cartridge and the atmospheric pressure is thus produced, the toner supply state from the developing cartridge may worsen or toner may be supplied more than necessary from the developing cartridge, degrading the quality of the image of printed matter.
Hitherto, an air hole is formed on an end face of the housing of a developing cartridge. The air hole is sealed by a seal member for preventing toner from flowing out, but allowing air to flow to put for making the pressure in the developing cartridge equal to the atmospheric pressure, as disclosed in Patent document 1.
[Patent Document 1]
JP-A-6-324568
However, pressure adjustment according to the structure wherein the housing is formed on the end face with an air hole as in the related art is attributed only to natural air communication caused by the difference between the pressure in the housing and the atmospheric pressure. Therefore, it takes some time until complete pressure adjustment is made. If the pressure in the developing cartridge rises accompanying a rapid temperature rise, there is also a possibility that a detrimental effect of excessive outflow of toner, etc., may be caused to occur before the pressure in the developing cartridge lowers.